Iron-clay interactions under hydrothermal conditions: Impact of specific surface area of metallic iron on reaction pathway
Résumé
The long-term evolution of minerals in contact with metallic iron is important in various domains such as the earth sc iences, materials science, cosmochemistry or industry. As an illustration, iron-clayey rock interactions are notable issues in the framework of secondary alteration processes in chondrites, or in the evolution of steel canister corrosion in projects for high-level nuclear waste repositories. In these contexts, interactions between the geological environment and metallic iron or engineered structures must be assessed with a high level of precision. Therefore, over the last decade, several experimental studies have focused on metallic iron-clay interactions showing the important relationship between the reaction progress and the iron/clay mass ratio. The present investigation demonstrates that, apart from this mass ratio, the specific surface area of metallic iron has a crucial influence, since it impacts the reaction pathway and the ambient physico-chemical parameters of the medium. For this purpose, two original continuous monitoring experiments were performed to measure pH and pressure in real-time, as well as analyze the gas and solution compositions, by bringing the same mass of (a) iron powder (S-iron = 0.07 m(2)/g) or (b) iron grains (Siron approximate to 0.001 m2/g) into contact with claystone (Callovo-Oxfordian claystone, Bure, France) at 90 degrees C for 3 months. Using iron powder, i. e. the more reactive cast iron (with a corrosion rate of 0.54 mmol/day for iron powder as against 0.01 mmol/day for iron grains), causes an Fe-enrichment of the clay particles, leading initially to the formation of new phases intermediate between interlayered illite-smectite and iron-rich serpentine, followed by conversion into odinite-greenalite. On the other hand, using iron grains make the clay compositions ``kaolinitic'' with a noticeable I. C. depletion. Meanwhile, the illite-smectite and quartz fractions of the claystone are destabilized, while the mineral transformations control the pH around 7 (+/-0.3) and prevent the formation of magnetite, thus contradicting the thermodynamic predictions. The present study, which involves in situ monitoring of pH and H-2 production, provides some important keys to obtaining better constraints on reaction mechanisms, kinetics and thermodynamic models, aimed at predicting accurate reaction paths and their long-term consequences.